dart_vm_binary should produce a standalone executable

[cbracken]

Previously, the Dart SDK had no support for building standalone executables. The two options were:

• Running from source
• Running from a binary snapshot tied to a particular VM version

Dart now supports building native ELF (Linux), Mach-O (macOS), PE (Windows) binaries. This drastically simplifies management and deployment of build products.

We should support emitting a standalone binary from dart_vm_binary. Ideally this should be the default, but it would be good to avoid breaking any existing users of the current bundle/snapshot format.

[luangong]

Any updates on this?